Underwater enclosure

Abstract

The present document discloses an underwater enclosure for holding a pressurised fluid for a variable buoyancy system, comprising: a watertight external shell for withstanding external underwater pressure; an airtight internal shell defining an internal chamber for holding the pressurised fluid and arranged for withstanding pressurised fluid pressure; wherein the internal and external shells are arranged in a spaced arrangement defining an intermediate volume between the internal and external shell. It is also disclosed a variable buoyancy system comprising the underwater enclosure, and an autonomous underwater vehicle comprising said variable system.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to an underwater enclosure, e.g., for a variable buoyancy system of an Autonomous Underwater Vehicle, AUV, a seabed lander, a underwater mining machine, among others.BACKGROUND

[0002] Usually, seabed landers are deployed from a surface vessel, descend to the sea floor, and are retrieved by shedding ballast [1]. For a lander to be reused, i.e., to descend to the sea floor, human intervention is required so more weight (ballast) is reloaded.

[0003] A Variable Buoyancy System was first considered so that a lander could be retrieved without having to shed ballast, could hover in mid-water and soft land on the seabed, and so minimize disturbance. Jamieson [2] describes the development of the concept and its application. A low power, compact VBS could also benefit underwater vehicles, especially unmanned underwater vehicles such as underwater gliders for a very energy efficient motion and / or for trimming the buoyancy, remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs) to improve the energy efficiency in vertical actuation to keep the depth.

[0004] Increased weight in underwater vehicles directly implies a greater need for floaters, necessitating a larger volume on the underwater vehicles to accommodate these floatation devices. As the weight of the vessel increases, more floaters are required to maintain buoyancy, which in turn demands additional space. This additional volume ensures that the vessel remains stable and afloat, allowing it to support the increased load effectively.

[0005] Document WO2005019021A1 [3] reveals a variable buoyancy device for controlling the buoyancy of unmanned underwater vehicles such as remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs). Herein, although the titanium spheres offer durability and are the best option from the several materials available, they are often too expensive, complicated to produce and heavy, affecting the buoyancy range and overall performance. On the other hand, although glass spheres can withstand high external pressures, they are not easily manufactured, in particular uniformly, and have the desired interfaces with the outside.

[0006] A ROV generally controls its position in the water column and lifts or lowers payloads by using vertical thrusters. When seawater properties vary from site to site, or a change of tooling is required, the ROV may require trimming to maintain slight positive buoyancy and level attitude. This can be time consuming and is usually done manually by lifting the ROV out of the water and changing or shifting ballast. A variable buoyancy capability enables a ROV to control its position, manipulate payloads and trim a vehicle in the water. An AUV does not require logistical support except for deployment and retrieval. The energy source is limited [4] and so its generally neutrally buoyant and moves vertically by lift generated from control surfaces as it moves forward. The limited energy source constrains the range of the vehicle and its data gathering capacity.

[0007] A variable buoyancy capability enables AUVs to use energy more efficiently and enhance its functionality, therefore some research groups have already developed functioning VBS [5, 6, 7, 8]. However, the existing variable buoyancy systems need to exponentially increase in weight / size with increasing pressure and depth; presenting constraints at the design / construction of underwater vehicles. High buoyancy variation volumes require large and heavy systems with a negative impact on marine operations, logistics, and transport.

[0008] At the moment, some glass semi-spheres are used as buoyancy elements in underwater systems, and used to hold electronics at pressures below atmospheric pressure, in order to keep the spheres attached to the surface. However, they cannot have internal pressure, as this causes a force proportional to the area times (pressure diff), which easily gives a very large value that is difficult to oppose by mechanisms for fixing the semi-spheres.

[0009] These facts are disclosed in order to illustrate the technical problem addressed by the present disclosure.GENERAL DESCRIPTION

[0010] The present document discloses an underwater enclosure for holding a pressurised fluid for a variable buoyancy system, comprising: a watertight external shell for withstanding external underwater pressure, i.e. the surrounding environment; an airtight internal shell defining an internal chamber for holding the pressurised fluid and arranged for withstanding pressurised fluid pressure; wherein the internal and external shells are arranged in a spaced arrangement defining an intermediate volume between the internal and external shell.

[0011] The present disclosure reduces the dependency on traditional buoyancy materials, such as syntactic foam, and achieves a significantly lighter structure. This solution minimizes the amount of extra buoyancy material required to achieve neutral buoyancy, leading to a more compact and efficient structure with a reduced overall system volume.

[0012] The optimized ratio of the weight of the pressure housing to its structural performance, results in a further reduction of system weight and volume compared to existing solutions, enhancing performance and adaptability across a range of underwater applications.

[0013] In an embodiment, the pressurised fluid comprises a gas and a non-compressible fluid and the underwater enclosure comprises a fluid exchange port for exchanging the gas and / or the non-compressible fluid between the internal chamber and an exterior fluid connector of the external shell, in particular the gas being air and the non-compressible fluid being oil or seawater.

[0014] In an embodiment, the fluid exchange port extends through the internal shell, intermediate volume and external shell for exchanging the gas and / or the non-compressible fluid between the internal chamber and the exterior fluid connector of the external shell.

[0015] In a further embodiment, the fluid exchange port comprises a floating port for exchanging the gas between the internal chamber and the exterior fluid connector of the external shell.

[0016] In an embodiment, the fluid exchange port comprises a ballast weighted port (108) for exchanging the non-compressible fluid between the internal chamber and the exterior fluid connector of the external shell.

[0017] In an embodiment, the underwater enclosure comprises a gas exchange port (101) for exchanging gas between the intermediate volume and an exterior gas connector of the external shell, in particular for drawing gas from the intermediate volume to obtain vacuum in the intermediate volume.

[0018] In an embodiment, the external shell comprises two hemispheres, in particular two hemispheres brought together and kept together by a lower pressure of the intermediate volume relative to an exterior of the external shell.

[0019] In an embodiment, the internal shell is a sphere, in particular comprising two hemispheres integrally forming the sphere, in particular the underwater enclosure being arranged such that the two hemispheres are brought together and kept together by a lower pressure of the intermediate volume relative to standard, or near-average, atmospheric pressure at sea level (101 KPa or 1 atm).

[0020] These provide greater manufacturability, and a guarantee of more homogeneous surface that a hollow sphere.

[0021] In an embodiment, the two hemispheres are soldered or strongly bounded to each other, to form the sphere.

[0022] In an embodiment, the external shell is made of glass, in particular borosilicate glass, bk-7 glass, sapphire glass, or combinations thereof.

[0023] In an embodiment, the internal shell is made of metal, in particular titanium or steel, a fibre-composite, or a combination thereof.

[0024] In an embodiment, the underwater enclosure further comprises a sealing ring positioned between the two hemispheres.

[0025] In an embodiment, the sealing ring comprises a flange for providing support and fixing points.

[0026] In an embodiment, the gas of the pressurised fluid is air, nitrogen, helium, hydrogen, or a combination of these; and the non-compressible fluid of the pressurised fluid is water, e.g., from the external environment, or oil (i.e. hydraulic fluid, hydraulic oil).

[0027] It is further disclosed, a variable buoyancy system comprising the underwater enclosure herein disclosed.

[0028] In an embodiment, the variable buoyancy comprises a plurality of underwater enclosure herein disclosed.,

[0029] It is also disclosed, an autonomous underwater vehicle comprising the variable buoyancy system herein disclosed.

[0030] In an embodiment, the variable buoyancy system further comprises at least one pump for driving the pressurized fluid into or out of the internal chamber of the underwater enclosure; preferably the pump is actuated via at least one actuator, in particular via a piston.

[0031] In an embodiment, the variable buoyancy system further comprises an external reservoir comprising pressurized fluid, in particular the pressurised fluid comprising a gas and a non-compressible fluid, wherein the reservoir is a flexible membrane reservoir comprising non-compressible fluid, further in particular non-compressible fluid being oil

[0032] In an embodiment, the intermediate volume is arranged for providing a temperature isolation of the pressurised fluid in the internal chamber.

[0033] In an embodiment, the internal chamber is a rigid chamber.

[0034] In an embodiment, the underwater enclosure further comprises a support between the external shell and the internal shell thus providing support of the internal shell.

[0035] In a particular embodiment, the support is present in only one of the glass hemispheres of the external shell, preferably the inferior or bottom part.

[0036] In an embodiment, the space between the external and the internal shells there is at a negative pressure relative to the underwater pressure, preferably the pressure is lower than 101 Kpa (1 atm), this preferential negative pressure allows for the two-sphere arrangement to hold together and has the unexpected result of thermally isolating the internal chamber.

[0037] In an embodiment, the external chamber is made of glass, which is a durable material capable of withstanding high pressures and corrosion encountered in deep-sea environments.

[0038] Due to the above-mentioned technical features, the variable buoyancy system is lighter than SOA systems and requires less extra buoyancy materials, such as syntactic foam, to turn the system neutral buoyant, which reduces the total volume of the buoyancy system. The weight of the pressure housing varies considering the characteristics of the vehicle and the operation depth, in this case this ratio was reduced. Furthermore, it uses a readily available material, glass hemispheres, at a cheaper cost than titanium spheres.

[0039] This two-shells arrangement represents an important solution for AUVs or other underwater systems, offering enhanced durability and efficiency while reducing logistical challenges associated with traditional single-shell systems.BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The following figures provide preferred embodiments for illustrating the disclosure and should not be seen as limiting the scope of invention. Figure 1: Schematic representation of an embodiment of an underwater enclosure. Figure 2: Schematic representation of an embodiment of an underwater enclosure. Figure 3: Schematic representation of an embodiment of an underwater enclosure. DETAILED DESCRIPTION

[0041] The present document discloses a configuration that allows the development of a variable buoyancy system (VBS) for high pressure in which the deposit weight does not scale with pressure (depth).

[0042] It comprises replacing the traditional pressure housing (typical metal: stainless steel, titanium or aluminium or composite materials) that hold the liquid and air in the VBS for a new housing as disclosed.

[0043] It is disclosed an underwater enclosure for holding a pressurised fluid for a variable buoyancy system, comprising: a watertight external shell (104) for withstanding external underwater pressure, i.e. the surrounding environment; an airtight internal shell (107) defining an internal chamber (102, 106) for holding the pressurised fluid and arranged for withstanding pressurised fluid pressure; wherein the internal and external shells are arranged in a spaced arrangement defining an intermediate volume (109) between the internal and external shell.

[0044] In an embodiment, the underwater enclosure comprises a glass sphere, preferably formed by two half spheres, that can handle high external pressures (for high depth >6000 m or even full depth, 11000 m), see Table 1, but cannot support internal pressure that would split the two spheres; and a thin metal sphere, preferably 0.5-2 mm, inside of the glass sphere with a diameter smaller than the internal diameter of the glass sphere, that can handle internal pressures required by the VBS operation (0.2 to 15 bar).

[0045] The thin metal sphere cannot hold external pressure solely, but since it is mounted inside of the glass sphere, in an embodiment it will be at least at the same pressure as the air in the small gap between the two spheres. This pressure will be lower than the atmospheric one to provide an adequate connection between the two spheres.

[0046] In an embodiment, the glass hemispheres are configured to withstand high external pressures up to 0.13 Kpa (6000 mmHg) and an internal pressure lower than atmospheric pressure, thus ensuring that the two hemispheres are joined together.

[0047] In an embodiment, the glass hemispheres comprise anti-sliding supports to prevent the hemispheres from sliding apart.

[0048] In an embodiment, the hemispheres are made of metal, preferably further comprising a flange around the edge of the hemisphere, which allows the two hemispheres to be clamped together. This flap is configured to withstand the force exerted by the internal pressure.

[0049] Figure 1, shows a schematic representation of an embodiment of a underwater enclosure, wherein 100 represents an assembled underwater enclosure, 101 represents a gas exchange port, 102 represents a space occupied by air, 103 represents a floating port connected to sphere air in / outlet, 104 represents an external shell, 105 represents a fluid exchange port, 106 represents a volume of oil, 107 represents an internal shell, 108 represents a ballast weight, and 109 represents an intermediate volume between the internal and external shell.

[0050] Figure 2, shows a schematic representation of an embodiment of a underwater enclosure, being the non-compressible fluid an oil, and where101 represents a gas exchange port, 105 represents a fluid exchange port, 109 represents the external reservoir, preferably a flexible membrane for holding the oil that is pumped out to increase volume and gain buoyancy and 110 is the pressurized housing, preferably a cylinder, comprising an oil / water pump, manifold and valves, hydraulic control electronics, pressure sensors and / or pressure reducers.

[0051] Figure 3, shows a schematic representation of an embodiment of a underwater enclosure, being the non-compressible fluid seawater, and where 101 represents a gas exchange port, 105 represents a fluid exchange port, 110 is the pressurized housing, preferably a cylinder, comprising an oil / water pump, manifold and valves, hydraulic control electronics, pressure sensors and / or pressure reducers, 111 is a water filter, 112 is environment water inlet / outlet.

[0052] In an embodiment, the glass hemisphere 104 is configured for deep sea applications and having a thickness 10-60 mm, more preferably 10-30 mm.

[0053] In an embodiment, the vacuum port and the fluid exchange port (105) are located at a distance from the ends of the hemispheres for preventing cracking under hydrostatic pressure.

[0054] In an embodiment, the internal shell 107 is supported via the fluid exchange port 105 to the external shell 104.

[0055] In an embodiment, the internal shell is a rigid metallic sphere.

[0056] In an embodiment, the fluid exchange port is placed with a ballast weight for keeping a lower position in relation to the oil, thus ensuring the extraction of oil therein.

[0057] In an embodiment, the underwater enclosure further comprises a second fluid exchange port opposite to the fluid exchange port with a ballast weight in order to promote an easier access to an air region inside the internal chamber.

[0058] In an embodiment, the internal and / or external shell are cylindrical, conical, parallelepipedal or spherical shaped. The spheric shape is designed to maximize the ratio of internal volume to surface area. Said shell is configured such that the inner and outer shapes deviate minimally from each other, optimizing structural integrity while maximizing internal volume capacity. The non-spherical geometries of the shell allows for efficient utilization of space within constrained environments, enhancing functionality and performance in applications where volume-to-surface ratio is critical.

[0059] In an embodiment, the fluid exchange port has multiple independent connections, for example one for the gas and another for the non-compressible fluid.

[0060] In an embodiment, a variable buoyancy system comprises: a manifold, a set of electro valves, pressure reducing valve and high pressure pump for oil displacement between the internal chamber and an external reservoir, being the oil the non-compressible fluid; and a controller electronic system and an actuator, preferably an electric motor or hydraulic actuator, for controlling the movement of the pump, i.e. the rate of oil pumping as well as controlling the manifold valves for commuting the direction of fluid pumping or holding the pressure for controlling electromechanically the device.

[0061] Alternatively, the variable buoyancy system comprises: a manifold comprising different paths for the fluid and hold a set of electro valves for controlling the circulation path; electro valves; pressure reducer / limiters due to the limits in the inlet pressure of the pumps and other hydraulic components (mainly for reducing / limiting the external ambient pressure in deep applications); a pump with associated electric motor (or hydraulic motor); at least one pressure sensor to measure the internal and external pressure; and an electronic controller for controlling the pump motor and electro valves, and receiving the measurement from the at least one sensor.

[0062] The buoyancy system can have multiple underwater pressure enclosures, and allows to scale buoyancy and / or provides pitch and roll control. The buoyancy system can have multiple pumps, for redundancy or scale pumping flow, and for attitude control, by pumping between enclosures controlling the buoyancy in different positions of the system.

[0063] In the case of having specific pumps for transfer of non-compressible fluid between different enclosures, those pumps do not need to support high pressures, only the maximum pressures inside of the enclosures (<15Bars), but can have higher pumping flows.

[0064] The piston displacement creates a pressure difference that drives the oil (or any non-compressible fluid or seawater from external environment) to flow either into the internal chamber 102, 106 (decreasing buoyancy) or out of the chamber into the reservoir (increasing buoyancy), depending on the direction of the piston's movement or the configuration of the electro valves open.

[0065] In an embodiment, the variable buoyancy system comprises at least one pump for driving the pressurized fluid into or out of the internal chamber of the underwater enclosure; preferably the pump is actuated via at least one actuator.

[0066] In an embodiment, the variable buoyancy system further comprises an electronic controller configured to receive a predetermined depth of the AUV and / or real-time AUV data, e.g. pressure, depth data, buoyancy level, environmental conditions, and to control the fluid pumping flow to achieve the total buoyancy set point defined by the AUV, i.e. the buoyancy of the AUV, via the actuator corresponding to a predetermined depth of the AUV. The electronic controller allows to maintain optimal stability and manoeuvrability of the AUV during underwater operations.

[0067] It is also disclosed, an operational method of the AUV comprising emitting a signal for retrieving of the AUV, in particular an acoustic signal, or a watchdog mechanism that if the main systems fail to interact in a predetermined time frame, the system becomes buoyant, or the AUV in case of some failure order VBS to become buoyant.

[0068] Allows the development of underwater vehicles / systems for deep sea that require fast buoyancy adaptation or large buoyancy changes.

[0069] This can be used to create vertical movement, like in glider-type vehicles, allowing large scale glider or hybrid gliders and propelled, or to create new vehicles for transport of equipment / material from / to sea bottom in a more energetically efficient way. Table 1 - The exterior shell material of and related propertiesType of exterior material Glass BK-7 Glass BK-7 Glass BK-7 Titanium Titanium Titanium Depth rating 600040006000600040006000Limit pressure (dBAR) 1200080001200012000800012000External Diameter (mm) 1061106150810611061508Wall tickness (mm) 332716342316Gap between spheres 886Internall sphere Wall tickness (mm) 221Total Volume (Litres or dm 3< ) 625,4625,468,6625,4625,468,6External Sphere Internal volume 515,8534,756,5512,7547,556,5Internal Sphere external volume 491,3509,652,3512,7547,556,5Internal sphere internal Volume -Volume usable (Litres or dm^3) 485,3503,451,6512,7547,556,5Volume Usable with Usage of 0% to 90% Fluid (Litres or dm^3) 436,8453,146,5461,4492,850,8Aba H (mm) 303030Delta Diam Max 252515Delta Diam Neg 252515Weight external sphere Join 0,00,00,044,344,312,7Weight external sphere 275,1227,730,6499,3344,953,9Weight internal sphere 36,036,94,00,00,00,0Estimated Total Housing weight (Kg) (includes weight of all housing parts for a functional VBS housing) 311,1264,634,6587,9433,579,4Ratio Variation Buoyancy / Estimated total Housing weight 140,41%171,27%134,34%78,49%113,66%64,03%System weight (Kg) underwater for neutral at 50% 44,2-12,36,2305,9132,448,3Extra Foam volume (dm^3) needed to be neutral at 50% (0.52 density) 94-2613651282103Total system Volume (Sphere, syntatic Foam) (dm 3< ) 719,3599,181,81 276,2907,1171,4Ratio Variation Bouyancy / total system Volume 60,72%75,63%56,79%36,15%54,32%29,65%

[0070] With the disclosed solution, it is achieved a significantly lighter device for the same volume, offering enhanced efficiency and versatility in buoyancy control. This reduction in weight allows for greater manoeuvrability and reduced operational costs, making it ideal for marine applications where fuel efficiency is crucial. Additionally, it allows for the use of either oil or environment seawater to adjust buoyancy as needed. This flexibility ensures an effective stabilization and operations more efficient. This solution also allows to build a more compact and cheaper systems, that can have the same size as the already known in the art but have a higher buoyancy variation capability that can lead to a more efficient and cost-effective transportation of material from the seabed.

[0071] Surprisingly, by employing a two-shells arrangement, it becomes feasible to achieve remarkable resilience in deep-sea environments. The internal sphere is configured to withstand high internal pressure (originated from the compression of the air when the non compressible fluid is pumped in) independently of external conditions. Simultaneously, the external sphere is designed to support significant external pressure, irrespective of internal conditions. This innovative approach effectively distributes pressure loads between the two shells, removing the necessity for an excessively large and heavy single shell. As a result, the overall weight and size of the structure are minimized without compromising its ability to withstand the demanding conditions of deep-sea operations.

[0072] This solution also relates to a pressure vessel for deep sea application, capable to hold within fluids at a broad range of pressures, see Table 1. It also presents a relation of effective holding volume versus its total weight better than any other existing approach, thus providing a particularly optimized fluid container for variable buoyancy systems.

[0073] The solution herein presents comprises several advantages namely: being lighter and smaller than the state of the art underwater enclosures, it is more resistant to higher depths, up to 11000m deep; possess an higher pressure resistance for the same displaced volume and higher volumes of buoyancy variation. Thus, providing smaller sized deep-sea vehicles with higher buoyancy volume variation (applicability and usefulness to a wider range of underwater vehicles).

[0074] The term "comprising" whenever used in this document is intended to indicate the presence of stated features, integers, steps, components, but not to preclude the presence or addition of one or more other features, integers, steps, components, or groups thereof.

[0075] The disclosure should not be seen in any way restricted to the embodiments described and a person with ordinary skill in the art will foresee many possibilities to modifications thereof. The above-described embodiments are combinable.

[0076] The following claims further set out particular embodiments of the disclosure.References

[0077] [1] P. M. Bagley, I. G. Priede, A. J. Jamieson, D. M. Bailey, E. J. V. Battle, and C. Henriques C2004. "Lander techniques for deep-ocean biological research" Underwater Technology, vol 26 no.1, pp3-11, 2004. [2] A. Jamieson, "Autonomous lander technology for biological research at mid-water, abyssal and hadal depths", Ch 5, PhD Thesis, University of Aberdeen, Scotland, 2004. [3] P. M. Bagley, M. A. Player, A. J. Jamieson, "A buoyancy control system", Patent No WO2005019021, 2005-03-03, 2005. [4] G. Griffiths, J. Jamieson, S. Mitchell, K. Rutherford, "Energy storage for long endurance AUVs", Proceedings of ATUV Conference, IMarEST, London, 16-17 March, pp. 8-16, 2004. [5] M. Worall, AJJamieson, A. Holford, etc, (2007) "A Variable Buoyancy System for Deep Ocean Vehicles", Oceans 2007-Europe, pp.1-6. [6] Wen-de Zhao, Jian-an Xu and Ming-jun Zhang (2010) "A Variable Buoyancy System for Long Cruising Range AUV" 2010 International Conference on Computer, Mechatronics, Control and Electronic Engineering (CMCE). [7] "TURTLE - A robotic autonomous deep sea lander" (2016) Eduardo Silva, Alfredo Martins, José Almeida, Hugo Ferreira, António Valente, Maurício Camilo, António Figueiredo and Cláudia Pinheiro. Proceedings of the OCEANS'16 MTS / IEEE Monterey conference, Monterey, California, USA, 19-23 September, 2016. DOI: 10.1109 / OCEANS.2016.7761262 [8] "TURTLE Robotic Lander in the context of REP2022 military exercise" (2023) Alfredo Martins, José Almeida, Carlos Almeida, Bruno Matias, António Ferreira, Diogo Machado, Hugo Ferreira, Ricardo Pereira, Eduardo Soares, Pedro André Peixoto and Eduardo Silva. OCEANS23 - Limmerick

Claims

1. An underwater enclosure for holding a pressurised fluid for a variable buoyancy system, comprising: a watertight external shell (104) for withstanding external underwater pressure; an airtight internal shell (107) defining an internal chamber (102, 106) for holding the pressurised fluid and arranged for withstanding pressurised fluid pressure; wherein the internal and external shells are arranged in a spaced arrangement defining an intermediate volume (109) between the internal and external shell.

2. The underwater enclosure according to the previous claim wherein the pressurised fluid comprises a gas and a non-compressible fluid and the underwater enclosure comprises a fluid exchange port (105) for exchanging the gas and / or the non-compressible fluid between the internal chamber and an exterior fluid connector of the external shell, in particular the gas being air and the non-compressible fluid being oil or seawater.

3. The underwater enclosure according to the previous claim wherein the fluid exchange port (105) extends through the internal shell, intermediate volume (109) and external shell (104) for exchanging the gas and / or the non-compressible fluid between the internal chamber and the exterior fluid connector of the external shell.

4. The underwater enclosure according to the previous claim wherein the fluid exchange port (105) comprises a floating port (103) for exchanging the gas between the internal chamber and the exterior fluid connector of the external shell.

5. The underwater enclosure according to claim 3 or 4 wherein the fluid exchange port (105) comprises a ballast weighted port (108) for exchanging the non-compressible fluid between the internal chamber and the exterior fluid connector of the external shell.

6. The underwater enclosure according to any of the previous claims comprising a gas exchange port (101) for exchanging gas between the intermediate volume and an exterior gas connector of the external shell, in particular for drawing gas from the intermediate volume to obtain vacuum in the intermediate volume.

7. The underwater enclosure according to any of the previous claims wherein the external shell (104) comprises two hemispheres, in particular two hemispheres brought together and kept together by a lower pressure of the intermediate volume relative to an exterior of the external shell, in particular the underwater enclosure being arranged such that the two hemispheres are brought together and kept together by a lower pressure of the intermediate volume relative to standard, or near-average, atmospheric pressure at sea level.

8. The underwater enclosure according to any of the previous claims wherein the internal shell is a sphere, in particular comprising two hemispheres integrally forming the sphere.

9. The underwater enclosure according to any of the previous claims wherein the external shell (104) is made of glass, in particular borosilicate glass, bk-7 glass, sapphire glass, or combinations thereof; and / or the internal shell is made of metal, in particular titanium or steel, a fibre-composite, or a combination thereof.

10. The underwater enclosure according to any of the claims 7- 9 further comprising a sealing ring positioned between the two hemispheres.

11. The underwater enclosure according to any of the claims 2-10 wherein the gas of the pressurised fluid is air, nitrogen, helium, hydrogen, or a combination of these; and the non-compressible fluid of the pressurised fluid is water or oil.

12. A variable buoyancy system comprising the underwater enclosure of any of the claims 1-11.

13. The variable buoyancy system of the previous claim further comprising at least one pump for driving the pressurized fluid into or out of the internal chamber of the underwater enclosure; preferably the pump is actuated via at least one actuator.

14. The variable buoyancy system of the claims 12 and 13 comprising an external reservoir comprising pressurized fluid, in particular the pressurised fluid comprising a gas and a non-compressible fluid, wherein the reservoir is a flexible membrane reservoir comprising non-compressible fluid, further in particular non-compressible fluid being oil.

15. An autonomous underwater vehicle comprising the variable buoyancy system of any of the claims 12-14.

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